The present invention relates to an R--Fe--B-based rare earth permanent magnet, wherein R is one or more of rare earth elements including Y (yttrium), and a production method thereof.
A rare earth permanent magnet, in particular, an R--Fe--B-based, sintered permanent magnet has been applied to a wide variety of fields due to a high performance thereof.
The R--Fe--B-based, sintered permanent magnet has a metal structure basically composed of three phases of R.sub.2 Fe.sub.14 B phase (main phase), RFe.sub.7 B.sub.6 phase (B-rich phase) and R.sub.85 Fe.sub.15 phase (R-rich phase). Generally, the R--Fe--B-based, sintered permanent magnet is inferior to an Sm--Co-based, sintered permanent magnet in corrosion resistance because of the presence of a rare earth element-rich phase and the three-phase metal structure. The poor corrosion resistance has been one of the drawbacks of the known R--Fe--B-based, sintered permanent magnet from the time of development to now.
Although the corrosion mechanism of the R--Fe--B-based, sintered permanent magnet has not been established, some report says that the corrosion proceeds with anodization of R-rich phase because the corrosion generally starts from R-rich phase. In fact, the amount of R-rich phase is reduced with decreasing content of rare earth element, and as a result thereof, the corrosion resistance of the R--Fe--B-based, sintered permanent magnet is improved. Therefore, one method for improving the corrosion resistance is to reduce the content of rare earth element.
A sintered rare earth magnet may be typically produced by a powder metallurgical method, for example, by melting and casting alloy metals for the magnet to form an alloy ingot, pulverizing the ingot to alloy powder, compacting the alloy powder to form a green body, sintering the compact body, heat-treating the sintered body and then working it. Since the alloy powder obtained by pulverizing an ingot has a high chemical activity because of a high content of rare earth element, the rare earth element is oxidized upon exposure to the atmosphere to result in increased oxygen content in the alloy powder. Therefore, a part of rare earth element is consumed to form a rare earth oxide to give a sintered body having a reduced content of magnetic rare earth element which contributes to magnetic properties of the sintered magnet. To compensate for the consumption of rare earth element and attain a practically sufficient level of magnetic properties, for example, a coercive force (iHc) of 13 kOe or higher, the content of rare earth element in the R--Fe--B-based, sintered permanent magnet is necessary to be increased. Practically, the rare earth element is added in an amount exceeding 31 weight %.
As mentioned above, the addition amount of the rare earth element should be decreased in view of improving the corrosion resistance, while be increased in view of attaining practically sufficient magnetic properties. Due to this antinomic requirement, a rare earth permanent magnet simultaneously having both a sufficient corrosion resistance and sufficient magnetic properties has not been obtained.